1. Field of the Invention
This invention relates to a gamma correction circuit, and more particularly to a gamma correction circuit for a digital video signal.
2. Prior Art
The relationship between a grid signal voltage and a luminous output of a color image receiving tube is not linear. The luminous output is proportional to, e.g., the 2.2-th power of an input signal applied to the grid. For this reason, when a signal from a video camera is applied to a color image receiving tube as it is, not only the luminance of an image on the screen but also its hue and/or saturation vary greatly. To prevent this, a technique is employed to pass an input signal, prior to applying it to the color image receiving tube, through a gamma correction circuit having an input/output characteristic such that an output signal is equal to, e.g., one 2.2-th power of an input signal, thus allowing the overall characteristic to be linear. It is sufficient to insert the gamma correction circuit immediately before the signal is applied to the grid of the color image receiving tube. However, due to economic and stability considerations, such a gamma correction circuit is not provided in association with a color image receiver of a home electronic apparatus, but is provided at the transmitting side, i.e., in a video camera.
In recent years, also with respect to video cameras, digital video signal processing technology to digitally process video signals has been widely used, and the above-mentioned gamma correction circuit has been constituted with a digital circuit.
For example, there is known a so-called mapping system which stores in advance a gamma correction characteristic into a memory and reads out from the memory a gamma corrected digital video signal. Namely, an approach is employed to store in advance an output signal level of the input/output characteristic for the gamma correction into the memory, to read out that output signal level from the memory by using an input signal level as a readout address for the memory, to use this output signal as a gamma corrected signal.
Further, as shown in FIG. 1, for example, there is known a gamma correction circuit adapted to perform gamma correction using a polygonal line approximating a desired gamma correction characteristic. Namely, as shown in FIG. 2, there is an approach to realize characteristics of respective segments of a polygonal line at respective operation circuits 1.sub.1 to 1.sub.n comprised of multiplier circuits and adder circuits, etc., to compare outputs from the respective operation circuits 1.sub.1 to 1.sub.n at a comparison circuit 2, to select the minimum value, thus providing a gamma corrected digital video signal.
In the above-described gamma correction using the mapping system, it is necessary to store all points on a curve (Y=X.sup.(1/r)) indicating the gamma correction characteristic. As a result, a memory of a when an approach is employed in order to realize high tone e.g., to increase the number of bits of a digital video signal, a memory of yet a larger capacity is required. Particularly, in the case of constructing the gamma correction circuit as an integrated circuit, e.g., by a so-called gate array, the increased memory capacity is an obstruction. Further, in the configuration of the memory, there is generally employed a scheme in which 8 bits are taken as one byte and in which the carrying out of a write/read of data is performed using a byte unit. Accordingly carry out write/read of data every byte. Accordingly, when the number of bits of a digital video signal is above 8, use efficiency of the memory is poor.
Moreover, in the case of altering the gamma correction characteristic for correspondence with different gamma correction characteristics, e.g., at every broadcasting station, the entirety of the memory contents must be rewritten. However, since the capacity of such a memory is large, much time is consumed in preparing data, in writing prepared data into the memory, or the like.
Further, in the case of the gamma correction circuit of the above-described configuration shown in FIG. 1, if the number of lines is increased for improving the precision of the polygonal line approximation, then the number of operation circuits increases accordingly, resulting in a problem in that the circuit scale becomes large. Furthermore, when the number of lines is above a certain value, it is difficult to realize a gamma correction circuit. In addition, it is necessary to alter characteristics (coefficients a.sub.i, b.sub.i : i=1 to n) of all the operation circuits 1.sub.1 to 1.sub.n in order to vary the gamma correction characteristic. However, this is impossible from a practical point of view.